Too much smoke compared to the source mesh size
I can see your domain covers way more volume then the rocket and the jet itself. The minimal amount of smoke rendered would be the amount contained in one voxel, looking at the shape of the smoke I'd guess you don't have enough domain divisions for such a comparatively small emitter.


If the scene is totally static and the rocket does not move, you should try scaling the smoke domain in such a way it's the smallest size possible to achieve the effects you need, as calculating empty space, just wastes processing power and relative voxel size is smaller, therefore the shape of the smoke is more precise.

To know the correct domain size try baking first with lower resolution division so you can see the general shape of the sim, than increase the resolution until you get the results you want.

If you want to emit a specific amount of smoke you can keyframe the "use inflow" box


And if you want to add more realism you could consider switching the emiter from smoke to "fire + smoke"